1. Technical Field
The disclosure relates to a pneumatic tire. More particularly, the disclosure relates to a pneumatic tire having a different color mark such as a white letter indicated on a sidewall part.
2. Related Art
Marks such as characters, signs and figures are indicated on a sidewall part of a tire. White letter in which, for example, a white character is embossed on an original black ground of a tire is sometimes used as the mark, and such a tire is called a white letter tire.
The white letter tire is generally produced as follows. A white rubber part for mark formation is provided on a sidewall rubber, followed by vulcanization molding in the state that the surface of the white rubber part is covered with a cover rubber layer comprising a black rubber. The part forming a mark is raised by a concave part of a mold during vulcanization molding. The surface of the raised part is shaved. White rubber is exposed by shaving the surface of the raised part, thereby a white letter that is an embossed mark comprising a while rubber is formed (see JP-A-4-275136 (1992)).
The white letter tire is that a white letter is indicated on one sidewall part provided outside when a tire is attached to vehicles, and consequently a white rubber part is provided. However, a white letter is not indicated on other sidewall part, and therefore, a white rubber part is not provided. In general, a white rubber reinforced by a non-carbon black filler has rigidity lower than that of a black rubber reinforced by carbon black. In spite of that, in the conventional white letter tire, a white rubber part is provided over the whole circumference in a tire circumferential direction in a sidewall part on which a white letter is indicated. As a result, due to the difference in rigidity between the white rubber and the black rubber, the difference in rigidity occurs between sidewall parts of both sides, and this may result in the decrease in driving stability.
JP-A-2000-255223 discloses that a white rubber part and a black rubber part are alternately and adjacently arranged in a tire circumferential direction on a sidewall part in order to provide a tire that appears to be colored when rotating. However, JP-A-2000-255223 does not disclose a shape of a boundary surface between the white rubber part and the black rubber part in a tire circumferential direction. If the boundary surface between the white rubber part and the black rubber part is a plane surface vertical to the tire circumferential direction, it is predicted that difference in strain in the boundary surface between the white rubber part and the black rubber part is increased during running, and there is a concern regarding separation in the boundary surface due to the difference in strain.
JP-A-2008-56785 discloses a technology of providing a temperature-sensitive color-changing part on a sidewall part of a tire. However, the temperature-sensitive color-changing part is formed into a circular pattern over the whole circumference in a tire circumferential direction. Thus, the disclosure of JP-A-2008-56785 does not suggest the present invention.